driving one for 5 years/120k KM and counting.

mk6 gti is similar to GTI mk5 in many things, in fact the mk6 gti is like the facelift version of mk5
the mk5 gti uses a 2.0TFSI engine instead of the 2.0TSI of the mk6 gti, the mk6 one is a bit more powerful stock for stock (mk6 205whp vs mk5 185?whp) and at stage 1-2 tunes

their chassis are similar, dsg similar, interior the mk6 is more modern but many things are actually quite similar from the mk5. many things are interchangeable between them in terms of mods (roll bars, intercoolers, unibraces, brakes, rims etc). except for engine related ones (intake, fuel pump, turbo?, etc) but Mk6 Golf R and mk5 gti uses the same engine (with the R having stronger internals etc)

but one thing the mk5 really miss from the 6 is DCC where you can choose between comfort, normal and sport.

comparing mk5 gti and e90 325i, the 325i probably has better comfort, refinement, better highway cruiser and prestige and maybe boot space. but for performance, point and shoot in town driving, mod potential, fun to drive, handling (not sure if the M sport is better tho), fuel economy, the gti wins